1. Field of the Invention
The present invention relates to an optical disc and an optical disc drive. More particularly, the present invention relates to a near field optical disc and a near field optical disc reading apparatus.
2. Description of Related Art
With development of storage technology, an optical disc using such technique may have advantages of large storage capacity, easy preservation, long duration of preservation, low cost and non-easiness of damage of data, etc., and data stored in the optical disc can be easily read through an optical disc drive. Generally, a read and write method of the optical disc is to focus a laser light on the optical disc through an objective lens, so as to perform the read and write operation, and a storage capacity of the optical disc is determined based on an area size of the optical disc. In case that the area size of the optical disc is fixed, if the storage capacity thereof is required to be further increased, a recording density of the optical disc has to be improved, or a wavelength of the laser light has to be shortened. However, limited by a physical restriction of diffraction limit, increasing of the storage capacity of the optical disc is bottlenecked. To resolve such problem, a concept of a near field super-resolution structure is provided, by which a near field optical principle is used to greatly improve the storage capacity of the optical disc.
The near field optics is a novel optics theory. In 1928, a British E. H. Synge first proposed a concept of obtaining optical information within a near field range, i.e. within a distance that electromagnetic waves still not generate interference and diffraction, so as to obtain a high spatial resolution beyond the diffraction limit. Then, in 1956, an American O'keefe also proposed a similar concept of using a hole with a size far less than a wavelength of a light source to closely approach to a surface of an object to be detected, so as to detect the optical information. However, limited by an engineering art of that time, E. H. Synge and O'keefe could not proof such concept by experiment.
In 1972, E. A. Ash and G. Nicholes first verified such concept by experiment. In the experiment, a microwave with a wavelength of 3 cm is used to observe the object within a distance that a light fluctuation does not generate a diffraction phenomenon, so as to obtain a spatial resolution with about 1/60 wavelength. This experiment is a first verification of the near field optics theory.
Then, in 1992, the American AT&T and Bell Labs obtain a super-high density surface record from a magneto-optical CoPt multiplayer film via the near field optics method. A method thereof is to melt and extend an optical fiber to form a probe with a nano-scale for performing light sending and receiving, which can write record points with a diameter of 60 nanometers on the magneto-optical CoPt multiplayer film, and can read signals of the record points. An experiment result shows that a super recording density of 45 gigabits can be achieved within each square inch.
A near field optical disc reading apparatus manufactured by the AT&T and Bell Labs according to a principle of the experiment has to integrate a light source into an optical pick-up head, so that light emitted from the light source and light reflected by the record points can all pass through the optical fiber probe, so as to achieve an effect of reading data according to the near field optical principle to overcome the diffraction limit. However, to make both the light emitted from the light source and the light reflected by the record points pass through the optical fiber probe with a tiny diameter, when the light source is installed and aligned to the optical pick-up head, it has to be installed to a very accurate position. Therefore, a permissible alignment error thereof is very small, which may lead to a long fabrication time and a high fabrication cost of the whole data reading apparatus.